PURPOSE: To produce power at low cost by guiding the sea water through a water-intake tunnel having predetermined head into a water storage tank to produce power from the flow speed in said tunnel while increasing the air pressure in said tank to return the sea water to the ocean.

CONSTITUTION: A water storage tank 7 is drilled in the land 1 and a water- intake tunnel 5 having head against said tank 7 is provided at the water-intake port 3 through a gate valve 4. The bottom of said tank 7 is conducted with a drainage path 9 equipped with a drainage pump 8 while the tank top is conducted through an air pressure regulating valve 10 onto the ground. A generator 6 is provided in the way of said tunnel 5 and the valve 4 is opened to flow the water thus to produce power. When the water level in said tank 7 reaches to head H, a regulating valve 10 is functioned to increase the air pressure in said tank 7 to the level corresponding to head D, thereafter a gate valve for said pump 8 is opened to discharge the sea water in the tank 7 through a discharge path 9.